Osteoporosis is a pathologic state or disease involving some symptom or risk due to quantitative reduction in bone exceeding a certain degree. Major symptoms are spinal kyphosis, and fractures of dorsolumbar bones, vertebral centra, femoral necks, lower end of radius, ribs, upper end of humerus, and others. In bone tissue, osteogenesis and bone destruction by bone resorption are repeated with a good balance (bone remodelling); osteoblasts and osteoclasts play key roles in osteogenesis and bone resorption, respectively. Bone resorption surpassing osteogenesis, upon deterioration of the balance between osteogenesis and bone destruction by bone resorption, results in osteoporosis with a quantitative reduction in bone. Traditionally, bone resorption suppressors such as estrogens, calcitonin and bisphosphonates have been mainly used as prophylactic/therapeutic drugs for osteoporosis. However, these bone resorption suppressors fail to have a satisfactory effect in some cases, due to limitation on the subject or to uncertain efficacy. There is therefore a need for an osteogenesis promoter that serves as a prophylactic/therapeutic drug for osteoporosis to increase once-decreased bone mass.
Bone morphogenetic protein (BMP), isolated from decalcified bone, is the only group of protein factors known to be capable of ectopic bone induction. It is therefore useful as an osteogenesis promoter in bone healing, bone reconstruction etc. Also, because BMP directly promotes osteoblast differentiation, it is assumed to play a role as a coupling factor in bone remodelling, and is thought to be closely involved in bone metabolism. Also, it is known that the BMP content in bone matrix in aged animals has been considerably decreased, suggesting that BMP is profoundly involved in the maintenance of bone mass. This suggests that BMP is promising as a therapeutic drug for various bone diseases such as osteoporosis. However, because BMP is normally present in trace amounts in living body so that its supply is limited, and because BMP is a protein so that a problem arises in its administration, the target diseases to which it is applicable are limited.
In addition, BMP has been reported to possess an activity like that of neurotrophic factors [Journal of Cell Biology, Vol. 119, p. 1,721 (1992)]. Also, because it is known that the BMP gene is strongly expressed in brain tissue, and because BMP has been suggested as playing an important role in neural tube formation in embryogenesis, BMP is thought to be profoundly involved in the differentiation or functional maintenance of nerve cells.
Neurotrophic factors, a group of proteinous factors playing an important role in the survival and functional expression of nerve cells, include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). NGF promotes the differentiation and maturation of the sympathetic ganglion cells and dorsal spinal root ganglion cells of the neural tube in the peripheral nervous system and acts on the cholinergic neuron cells of the septal field (procephalic basal ganglia) in the central nervous system. NGF is essential for the maintenance of nervous function even after completion of ferentiation. BDNF acts on the dorsal spinal root ganglion cells and nodal ganglion cells in the peripheral nervous system but does not act on sympathetic ganglion cells. On the other hand, in the central nervous system, BDNF acts on the cholinergic nerve cells and GABA (.gamma.-aminobutyric acid)-acting nerve cells of the septal field, and the dopaminergic nerve cells of the mesencephalon. NT-3 is characterized by potent action on the sensory nerve cells derived from the neural plate, although its action overlaps those of NGF and BDNF in the peripheral nervous system.
Alzheimer dementia has been characterized by extensive disorder and loss of cerebral cortical nerve cells, as well as degeneration and loss of cholinergic neuron of basal forebrain, including the septum; NGF and new neurotrophic factors are considered as candidates for therapeutic drugs therefore. Also, for Parkinson's disease, a disease characterized by degeneration and loss of the mesencephalic dopaminergic nerve neuron, BDNF and GDNF (glial cell line-derived neurotrophic factors), neurotrophic factors for those nerve cells, are expected to serve as a therapeutic drug. Because these neurotrophic factors are proteins, however, their application are subject to limitation.
Osteoarthritis, a non-inflammatory disease based on articular cartilage degeneration, is irreversible and progressive, although its progression is slow. Spinal intervertebral degeneration is relatively common among males, while the incidence of knee joint degeneration is relatively high in females. Etiologically, systemic factors such as genetic predispositions, age, estrogen and obesity, and the local factor of mechanical load are involved. When articular cartilage begins to be destroyed by various causes, proteolytic enzymes, mainly metalloprotease and serine protease, are locally produced, whose action causes cartilage matrix lysis, resulting in cartilage cracking, abrasion, ulcer etc., which in turn lead to the exposure of the subcartilaginous plate and sometimes cause calcium pyrophosphate crystal deposition on the deformed articular cartilage surface. Clinical symptoms include pain, hydrarthrosis, limitation of range of joint motion, creaking sound and deformation. Although much remains unknown as to the mechanism of onset, it is known that production of collagenase and other matrix metalloproteases (MMPs) is induced by the cytokines produced by chondrocytes, macrophages and synovial cells, such as interleukin (IL)-1, IL-6 and tumor necrosis factor (TNF)-.alpha., resulting in the collapse of articular cartilage. Drugs that inhibit the metalloprotease production induction by these cytokines are therefore expected to be effective as prophylactic/therapeutic drugs for osteoarthritis; however, there are no known drugs with such action, and conventional chemotherapies comprise nothing more than symptomatic therapies such as oral or topical administration of sedative anti-inflammatory drugs, and intra-articular injection of articular cartilage-protecting drugs such as aqueous solutions of hyaluronic acid.